1. Field of the Invention
The present invention relates to a separation membrane module having solid-liquid separating hollow fiber separation membranes used in the field of purification applications including the preparation of service water, drinking water, industrial water and pure water and polluted liquid treatment applications including removal of pollution from river water and sea water, treatment of waste water, night soil excrement and sewage water and condensation of slurry such as sludge, and a method for manufacturing such a separation membrane module.
Particularly, the present invention relates to a separation membrane module and a method for manufacturing such a separation membrane module, suitable for the treatment of a large amount of water.
Further, the present invention relates to water treating apparatus and method using such a separation membrane module.
2. Description of the Related Art
A hollow fiber separation membrane serves to cause liquid to flow through a tube wall having a great number of small holes to separate solid components or particles from the liquid. Material of the tube wall may be, for example, polyethylene, polypropylene, polysulfone, polyvinyl alcohol and their derivatives, 4-ethylene fluoride resin or cellulose acetate. An outer diameter of the hollow fiber separation membrane is 0.4 to 5 mm, for example, an inner diameter of the hollow fiber separation membrane is 0.3 to 4 mm, for example, and a diameter of each small hole is 0.01 to 5 xcexcm, for example.
Since capacities required in a separation membrane module of a solid-liquid separation device are factors directly relating to water treatment amount and treatment efficiency such as wide separation area and good contact with fluid to be treated, for example, there have been investigated many hollow fiber membrane modules of a cylindrical type which comprises a bundle of hollow fibers arranged in a circle or concentric circles and has conventionally been used in the field of precision filtration applications. In this technique, the hollow fiber separation membranes are arranged in high density as much as possible and the number of hollow fibers are increased to obtain a condition that the hollow fibers are substantially in contact with each other, so that the surface area of the membranes is increased to increase the water treatment amount for each module.
However, if liquid to be treated having high density of suspended substance (for example, suspension in aeration tank used for sewage treatment process) is filtered by using such a hollow fiber separation membrane module, the suspended substance enters between the large number of hollow fiber separation membranes and is trapped therein. Consequently, the suspension cannot be supplied to the trapped zones, with the result that a plurality of hollow fiber separation membranes are apt to form stick-shaped bundles (such phenomenon is referred to as xe2x80x9cinter-fiber cloggingxe2x80x9d). If such bundles are formed, since it is difficult for the liquid to be treated to enter into and be in contact with the interior of the bundle, the effective filtering area of the module is reduced, thereby worsening the filtering ability.
When such a hollow fiber separation membrane module (in which hollow fibers cohere (or stick) together into a mass) is periodically subjected to membrane surface cleaning or back washing, it is not easy to restore the function of the module whose hollow fibers once cohered together into a mass; in some cases, the hollow fiber separation membranes may also be damaged. This problem becomes noticeable particularly when highly polluted water or a large amount of water is treated.
In order to solve such a problem, modules having various configurations have been developed. For example, there has been proposed a water treating apparatus of immersing type in which a bundle of hollow fibers are arranged like a rattan blind in a flat plane to form a module and a plurality of modules are disposed side by side in a bath to form rough density sections, for example, refer to JP-A-5-220356 (Japanese Patent Laid-open), etc. However, also in such modules, since the arrangement of the bundle of the hollow fibers creates partial density sections and the hollow fibers are partially in contact with each other, the above problem cannot be solved completely.
The present invention aims to provide a hollow fiber separation membrane module and a method for manufacturing such a module, in which a polluted substance is hard to be deposited between hollow fiber separation membranes, and any increase in pressure difference, reduction of flow rate and inter-fiber clogging can be prevented for a long time.
Further, the present invention aims to provide a method for manufacturing a hollow fiber separation membrane module, in which, when the hollow fiber separation membrane module is manufactured, a great external force is not required, and the hollow fibers or bundles of hollow fibers can be interconnected in a spacial relation.
An object of the present invention is to provide a hollow fiber separation membrane module of immersing type having a plurality of hollow fiber separation membranes extending into liquid to be treated and a water collecting portion connected to ends of the hollow fiber separation membranes.
In the module according to the present invention, an inner cylindrical hollow portion of each hollow fiber separation membrane communicates with inner spaces of water collecting plates provided in the water collecting portions, and hollow fiber separation membrane strings comprised of one to seven hollow fiber separation membranes are spaced apart from each other so as not to be in contact with each other and are disposed in the water to be treated in a loosened condition.
Preferably, the module according to the present invention has the following arrangements. That is to say, (a) two water collecting portions are spaced apart from each other, and the hollow fiber separation membrane strings extend between the two water collecting portions and both ends of each hollow fiber separation membrane string are connected to the respective water collecting portions; (b) there is further provided a cleaning means a (FIG. 1 or 6) for removing substances accumulated on surfaces of the hollow fiber separation membrane strings from such surfaces; (c) the cleaning means includes a vibration means capable of vibrating the hollow fiber separation membrane strings; (d) the cleaning means has an air dispersing device having air dispersing plates disposed on the water collecting plates and adapted to supply bubbles into the liquid to be treated; (e) the cleaning means has a fluid pressure generating means so that pressurized fluid is discharged from discharge openings formed in the water collecting plates to flow the fluid toward the surfaces of the hollow fiber separation membrane; and (f) the water collecting plate has a plurality of water collecting openings, and the water collecting openings of one module can be connected to the water collecting openings of another module so that two or more modules are integrally connected to each other.
A concrete embodiment of the hollow fiber separation membrane module of immersing type is shown in FIG. 1, and supplementary explanation is made regarding the hollow fiber separation membrane module of immersing type according to the present invention. However, in FIG. 1, an example in which the hollow fiber separation membrane module of immersing type has a rectangular arrangement is illustrated, this example is merely used for explanation of the invention, and, thus, the present invention is not limited to such an example; however, a cylindrical arrangement as shown in FIG. 6 can be applied to the present invention. Therefore, the present invention does not limit the configuration of the module.
Incidentally, in the following explanation, the hollow fiber separation membrane or the bundle of the hollow fiber separation membranes are also referred to merely as xe2x80x9chollow fiber membranexe2x80x9d, and the hollow fiber separation membrane module is also referred to merely as xe2x80x9chollow fiber membrane modulexe2x80x9d. Further, the hollow fiber separation membrane strings are constituted by one or more hollow fiber separation membranes.
In the hollow fiber separation membrane module of immersing type shown in FIG. 1, the hollow fiber separation membranes (2) (or the bundle of the hollow fiber separation membranes) are disposed between water collecting plates (3) of plate-shaped water collecting portions arranged on both ends of a module (1) in a loosened condition. Each water collecting plate (3) of the plate-shaped water collecting portion is provided with a water collecting opening having a socket (4) to be connected to a similar water collecting plate of another hollow fiber separation membrane module.
In order to connect the hollow fiber membranes (2) to the water collecting plates (3) in a spaced (or non-contact) relation, a single hollow fiber membrane (2) or a bundle of plural (two to ten-odd; preferably, four or five) hollow fiber membranes (2) are spaced apart from each other (so as not to be in contact with each other) in a flow of liquid W to be treated shown by an arrow in FIG. 1 at connection points between the hollow fiber membranes (2) and the water collecting plates (3), and the hollow fiber membranes (2) are connected to the water collecting plates (3) so that the hollow fiber membranes are spaced apart from each other such that the hollow fiber membranes are not in contact with each other even when they are mounted in the loosened condition. A distance between the hollow fiber membranes (2) on each water collecting plate (3) is about 0.5 to 10 mm, for example, and, in case of the bundle of hollow fiber membranes (2), such a distance is preferably greater and may be about 3 to 10 mm. Further, in case of the bundle of hollow fiber membranes, spacers may be provided in the vicinity of the connection points of each water collecting plate (3) so that the hollow fibers constituting the bundle are spaced apart from each other.
In the conventional hollow fiber membrane modules of flat type such as rattan blind, since the bundles of hollow fiber membranes are disposed side by side and are partially in contact with each other, when the membrane surface washing is effected, the membranes are in contact with each other to generate the inter-fiber clogging, with the result that the surfaces of the hollow fiber membranes cannot be cleaned sufficiently, thereby worsening the filtering efficiency.
In order to mount the hollow fiber membranes (2) in the loosened or relaxed condition, when the hollow fiber membranes are wound around two parallel winding bars, a distance between the parallel winding bars is adjusted to be slightly greater than a distance which consists of a length of the membranes needed to be connected to the water collecting plate and a length between the left and right water collecting plates (3) provided on both ends of the membrane module (1). The extent of the loosened or relaxed condition in the hollow fiber membranes (2) may be such that the ratio (y/x) between a first length (x) of the hollow fiber membranes (2) in a tightly extended condition and a second length (y) of the hollow fiber membranes (2) to be added to the first length (x) so as to loosened the membranes is, for an example, 5%. In the present invention, the hollow fiber membranes (2) are to be loosened when they are immersed in a liquid. However, since the hollow fiber membranes (2) generally have tendency to increase their length by their use or being immersed in a liquid, they are not always necessary to be loosened during manufacturing but may be loosened by some way after being manufactured.
In the present invention, there may be provided a hollow fiber membrane module in which a water collecting plate of a water collecting portion is provided at one side of the membrane module and hollow fiber membranes are open to the water collecting plate, and the hollow fiber membranes are bent back at the other side, or a hollow fiber membrane module in which the other ends of the hollow fiber membranes are closed. However, it is desirable that water collecting plates of water collecting portions are provided on both ends of the membrane module.
The present invention also provides a method for manufacturing a hollow fiber separation membrane module. The method according to the present invention includes a first step of rotating two winding bars disposed in parallel with a predetermined distance around central axes of the winding bars to wind a plurality of hollow fiber separation membrane strings each comprised of one or more hollow fiber separation membranes around the winding bars such that they are not in contact with each other through a predetermined longitudinal width along the winding bars, a second step of rotating the two winding bars around which the plurality of hollow fiber separation membrane strings were wound along the longitudinal direction of the winding bars together with the hollow fiber separation membrane strings to form hollow fiber separation membrane elements, and a potting step of potting the hollow fiber separation membrane elements on water collecting plates.
Preferably, the manufacturing method according to the present invention has the following arrangement. That is to say, (g) each winding bar is constituted by a rigid core member telescopically inserted into a flexible hollow tube; (h) each winding bar is provided at its surface with a winding groove for the hollow fiber separation membrane strings; (i) each winding bar is formed from thermoplastic material having a melting point lower than that of the hollow fiber separation membrane string; (j) the first step includes a step of controlling a ratio between the rotational speeds of the winding bars and the speed for shifting the hollow fiber separation membrane strings along the longitudinal direction of the winding bars to a predetermined ratio; (k) the second step for forming the hollow fiber separation membrane elements includes a step of rotating the two winding bars with the provision of spacers at positions of the two winding bars; (l) the second step for forming the hollow fiber separation membrane elements includes a step of rotating the two winding bars to give desired configurations to the hollow fiber separation membrane elements by forming bent portions at predetermined corresponding positions on the two winding bars; (m) the second step for forming the hollow fiber separation membrane elements includes a step of reshaping the hollow fiber separation membrane elements to desired configurations; (n) the potting step includes a step of fixing one end of the hollow fiber separation membrane elements by immersing the one end into a potting agent, forming opening portions of the hollow fiber separation membranes by cutting the potting portions, and connecting the opening portions to the water collecting plates; (o) the potting step includes a step of inserting a core having recesses into which ends of the hollow fiber separation membrane elements can be entered within a mold for forming the water collecting plate, inserting the ends of the hollow fiber separation membrane elements into the recesses of the core, and, after the mold for forming the water collecting plate is filled with material of the water collecting plate and the material is solidified, removing the core to form a water collecting space, and cutting the ends of the hollow fiber separation membrane elements, and communicating the openings of the hollow fiber separation membranes with the water collecting space; (p) the second step for forming the hollow fiber separation membrane elements includes a step of withdrawing the rigid cores from the winding bars before the two winding bars are rotated; and (q) the second step for forming the hollow fiber separation membrane elements includes a step of rotating the winding bars while hollow fiber separation membrane fixing agent is being poured into the winding grooves or after the hollow fiber separation membrane fixing agent is poured into the winding grooves, when the two winding bars are rotated.